Performing well (2,4)
I believe the answer is:
on song
'performing well' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for on song that I've seen before include "In superlative form" , "Performing well - in form (slang)" , "on top form?" , "Vocally" , "Performing well (colloq.)" .)